Refining of irons and steels



y 1964 YASUKIYO NISHIDA REFINING 0F IRONS AND STEELS 3 Sheets-Sheet 2 Filed Feb. 26, 1962 Treated x I00 Umtreated x100 1'15. 5

Added wit/c 0.04X/00 umeazdx/oo ref/41mg agent y 12, 1964 YASUKIYO NISHIDA I 3,132,936

REFINING 0F IRONS AND'STEELS Filed Feb. 26, 1962 s Sheets-Sheet 5 mu 93% sum 2 54 g: g E g 50 3 284 E E 46 E-6% E I5 42 WW sesa- 5" 5:; g 5 3432- E Q 3 E 3028- E E3; 2624- Added amount Treated with 0.08% Untreated refining agent 0.20% 0./0% Unrrea Zed Present invention United States Patent 3,132,936 REFINlNG OF RUNS AND STEELS Yasuhiyo Nishida, 6 l-chome, Fuse, .lapan Filed Feb. 26, 1962, tier. No. 175,903 Claims priority, application Japan Mar. 5, 1961 '1 Claim. (U. 753) The present invention relates to a method of refining irons and steels characterized in that a mixed compound of boric acid and metallic magnesium such as Mg (BO is added to the melt of cast iron or steel at a rate of 0.001 to 1% by weight as refining agent to improve the casting structure of steel and cast iron.

The principal object of the invention is to provide steel or cast iron having excellent mechanical properties with improved cast structure.

The steels and cast irons to which the refining of the invention can be applied are austentite steels, martensite steels, sorbite steels, ferrite steels and other killed steels, ordinary cast steels, refractory cast steels, stainless steels and the like ordinary steels, lower alloy steels, higher alloy steels and nodular graphite cast irons.

The manner of adding the refining agent of the invention may be done when the cast iron or steel is melted in a furnace or when the steel is in the molten state in a ladle or mould by adding a mixture of the compound consisting of Mg (BO and Mg(BO- either in the state of powders, granules, masses or by a suitable mix ture thereof at a rate of 0.001 to 1%, preferably 0.001 to 0.5%, or 0.001 to 0.3% by weight to the melt. The addition of the small amount of the refining improves the cast structure of various steels, such as austenite, ferrite, and alloy steels, nodular graphite cast irons and other cast irons. Also, the deterioration of steel structures, such as forging crack due to the raw material working, i.e. forging, rolling and the like workings, are prevented thereby improving both the mechanical properties and the yield of steel ingots and castings, and steels and cast irons of excellent qualities can be produced.

Various contrivances have heretofore been proposed to improve the structure of steel, yet the difference between the object and process of the addition agents which have heretofore been used and those of the invention will be described. Among the heretofore known addition agents, some are used for the purpose of alloying with steel and others are used forthe purpose of eliminating impure elements which give adverse efiects to steel, which agents are known as deoxidizing agents, desulfurizing agents, dehydrogenation agents for deoxidation, desulfurization and the like purposes. On the other hand, the addition agent used according to the method of the invention is neither intended to eliminate such imptue and adverse elements nor has the purpose of alloying with steel used for the special purpose only, but the method of the invention is based on an essentially diiferent idea from the known methods. The nature of addition agent according to the invention will be explained in detail below.

The addition agent, i.e. the refining agent to be used in the method of the invention consists of Mg (B0 and Mg(BO The results of thermal analysis on this agent using a thermobalance in the open atmosphere are shown in FIG. 1. As shown, the weight of the agent increased abruptly at 800 to 1,300" (1., the total increased weight arriving at up to 9% of the original weight. As may be judged from this phenomena the addition agent to be used in the method of the invention is stable from room emperature to 800 C. but it changes to an unstable compound at 800 to 1,300 C. and further reacts with oxygen to increase its weight. When the unstable compound within such a temperature range is added to the molten steel at a temperature higher than l,400 C., it combines with oxygen in the steel similarly to the other known 3,1323% Patented May 12, 1964 ice addition agents (deoxidation agents), but the reaction is not completed within a short time after the addition as in the other deoxidizing agents, and it takes a long time from the beginning of the reaction to the completion. During this period the addition agent has characteristics to continue the active reaction movement constantly as far as the reactive temperature of l,300 C. is given. This is a great difierence of the invention from various kinds of known addition agents. Such phenomena are due to the property of unstable compound which causes reaction at 800 to 1,300 C. As above described, the effect of the addition agent of the invention is characterized by the continuation of reaction period, possibility of reaction at a lower temperature and larger reaction energy if compared with the other elements or compounds. Accordingly, the activity due to the reaction has a greater eifect on the solidifying period and the temperature gradient of steel to improve the cast structure.

Now the process for improving the casting structure which is one of the objects of the method of the invention will be explained. When the steel is melted and it is in the molten state, if a very small amount, such as 0.001 to 1% by weight of the refining agent Mg (BO is added to the melt the agent commences the chemical reaction at once.

By this reaction the fluidity of the molten steel is remarkably improved. in this case, the molten steel treated with the refining agent continues the chemical reaction to improve the fluidity of steel and retards the commence ment of the solidification. The refining agent according to this invention acts over an extended time of the casting operation, which may, for example, be several hours in a large casting. The extended reaction time is due to the fact that the molten steel composition and the addition agent of the invention can not complete the chemical reaction within a short time. As the temperature de creases the steel arrives at the solidification temperature. However, since the reaction of the addition agent is still violent at this temperature, the start of the solidification is prevented. The molten casting is, therefore, overcooled (cooled below the normal solidification temperature while still molten) so that the interval between the commencement and completion of solidification is remarkably reduced. Also, the temperature gradient across the casting becomes so small that the normal progression of solidification of steel ingot from the surface and bottom layers toward its center as in the former processes is prevented. In the cast structure of the steel solidified under such condition no columnar crystal and dendritic structure occur in the macro structure but free crystals are formed over the total surface. in the micro structure of the austenite steel, the particles of austenite are made finer and in ordinary ferrite steels ferrite particles are fine and dispersed. Such phenomena are caused by the reaction energy of the addition agent in the melt. In the other processes it has recently been investigated to attain the object of segregation of free crystals and the method of producing a line grained structure by given continuous mechanical vibrations during the castiugprocess before the molten steel solidifies. On the other hand, according to the method of the invention it has been found that the cast structure of steel and cast iron can be improved by supplying the energy caused by the chemical reaction of the addition agent to the molten steel instead of the mechanical vibration to lengthen the reaction period of the melt until it solidifies. As to the yield of cast products, the shrinkage depression has heretofore occurred due to shrinkage at the central portion or top portion depending on the length or duration of the solidifying period and the temperature gradient, but according to the method of this invention there occurs a shrinkage depression having the same form or the same condition as if the top portion of the casting were heated after casting by means of riser thermit (exothermic agent) so that the secondary shrinkage in the form of pipe and the concentration of impurities in the central portion can be avoided, thereby greatly improving the yield of cast product of steel. The elongation and bending strength can be greatly improved by the fine grained structure of macro and micro. In the castings of ferrite steel, acicular structure does not occur in the as-cast structure, but there occurs such a structure of steel as if it has been recrystallized by annealing at 900 C. and also the same mechanical properties. In cast irons mechanical properties can be greatly improved by the efiects of above described reaction making rough grains of ferrite and perlite, thereby making the grain size of nodular graphite segregated between them smaller and increasing the number of grains.

For a further explanation of the invention reference is taken to the accompanying drawings, in which,

FIG. 1 represents a curve diagram of the thermal analysis of the refining agent according to the invention,

FIGS. 2(a) and (b) illustrate the state of blow holes occurring on the ingots when treated with the addition agent and untreated respectively,

FIGS. 3(a) and (1)) illustrate the microscopic struc tures of steels when treated with the addition agent and untreated respectively.

FIGS. 4(a) and (b) illustrate the other microscopic structures of treated and untreated samples respectively,

FIGS. 5(a) and (b) show the shrinkage depression on top and pipes of treated and untreated steel ingots respectively,

FIGS. 6(a) and (b) illustrate microscopic photos of cast structure of the treated and untreated steel ingots respectively, p

FIG. 7 represents comparative diagrams of elongation and reduction of area of treated and untreated steel ingots respectively,

FIGS. 8(a) and (b) are front and side view respectively of a pillow type test piece showing the condition of shrinkage depression on the top,

FIGS. 9(a) and (b) represent microscopic views of test pieces shown in FIGS. 8(a) and (b) respectively, and

FIGS. 10(a), (b) and (0) illustrate the microscopic views of the structures when added with 0.20% and 0.10% of the refining agent of the invention and no addition respectively.

The test examples of various kinds of steel castings obtainable by the process of the invention will be described in the following:

TEST EXAMPLE 1 Killed Steel Object: Control of Crystal grain, prevention of secondary pipe and orientated crystallization.

Method of testing:

C 009 P 0013 Mn 44 S 031 Si 10 Cu 013 The purifying agent of the invention at a rate of 0.015% by weight was added to the killed steel of the above composition in a ladle when it was tapped from an open hearth of 50 tons capacity. Allowing the reaction period of 5 minutes after the addition it was poured in a 100 kg. mould with riser, the time required from the beginning of addition to the completion of pouring being about 30 minutes.

RESULTS A. Condition of top and shrinkage depressi0n.'lhe steel ingot at the extreme end of the mould series poured with molten steel passed about minutes after the addition was taken as the samples.

The untreated steel ingot showed a little swell on the top, while the ingot treated with the refining agent of the invention showed a little depression. The sample was cut through the center of its top and the shrinkage de pression and the secondary pipe were investigated, the results of which are shown in FIG. 2.

B. Structure-The results of microscopic test of the sample taken from said top section are shown in FIGS. 3(a) and (b). As apparent from the photos the untreated sample shows the deposition of laminer perlite, while the sample treated with the refining agent of the invention shows the fine grained state of perlite so that the aggregation is remarkably prevented.

C. Structure and mechanical properties after rolling. The steel ingot treated with the refining agent of the invention was rolled to a wire of 5.5 mm. dia. and its section was tested by a microscope, the results of which are shown in FIGS. 4(a) and (b).

As evident from FIGS. 4(a) and (b) the sample treated with the invention shows the line grained state as compared with untreated sample even after the rolling.

Mechanical properties:

Tensile Yield Elonga- Breaking strength point tion, strength kgjmm. kgJmm. percent Break TS YP EL Treated by the invention"--- 42. 0 30. 0 3 .0 34 D 41.0 30.7 32. 5 34 41.7 28.1 31. 8 29 42.0 29.4 27. 7 30 It has been found that the sample treated by the invention has improved mechanical properties it compared with the untreated sample.

TEST EXAMPLE 2 Ordinary Cast Steel Object: Improvement in the quality of material and shortening of annealing. Method of testing:

C 0.20 S 0.01 Mn 0.68 Cu 0.15 Si 0.40 Al 0.04 P 0.01

When 10 tons of molten steel having the above composition are taken out of a basic electric furnace the refining agent of the invention was added into the ladle at a rate of 0.04% by weight of the melt and allowing the reaction time of 5 minutes after the addition. The melt was poured into each mould of the products and into the mould of pillow type test pieces about 15 minutes after the addition of the reagent.

The test results of the as-cast samples are as follows:

(1) Condition of the top and shrinkage depression: The melt was poured into moulds of 5" dia. x 2 length, and the shrinkage depression and secondary pipe at top were tested, the results of which are shown in FIGS. 5(a) and (b). As seen from the figures the sample ((1) treated by the invention showed no secondary shrinkage depression, while the untreated sample showed some secondary shrinkage depressions.

(2) The microscopic structures of the as-cast sample are as shown in FIGS. 6(a) and (b): As seen from the microscopic photos in FIGS. 6(a) and (b) taken from the pillow type test pieces the sample treated by the invention showed considerably fine grained structure without acicular structure as in the untreated sample which showed large acicular structure.

(3) Comparison of mechanical properties of as-cast condition and after annealing: The mechanical properties of the sample treated by the invention by the addition of 0.04% of the refining agent under the as-cast condition and those annealed at 650C. and 900 C. respectively are as follows.

Tensile Yield Elonga- Reduc- Hardstrength point tion, tlon of HB ness kg./mm. kgJmmfl percent area. B

As-cast state 46. O 24 30. 4 42. 0 126 120 Annealed at 650 0 50.0 26 0 34. 0 52.0 123 121 Annealed at 900 C. 51. 0 27 0 34.0 53.0 121 120 The treated sample showed even under the as-cast condition a great improvement in mechanical properties if compared with the untreated ones. The sample annealed at 650 C. to relieve strain showed the same results as that annealed at 900 C. and the as-cast sample and that annealed at 900 C. showed little difference in structure.

Mechanical properties due to the change in the amount of adding the refining agent of the invention: The relation between the quantity of the refining agent of the invention to be added and the mechanical properties was examined about several tensamples of C 0.20 to 0.25%, the results of which are shown in FIG. 7.

The effect on a common cast steel by the addition of the refining agent of the invention: If the refining agent of the invention is added to an ordinary cast steel at a rate of 0.04% the fluidity of the molten steel was remarkably improved and the structure and mechanical properties of as-cast products showed substantially the same effect as that which was subjected to the annealing at 900 C. of the former and moreover, in a large size of casting the damage of crack when cutting the riser did not appear and the secondary pipe and shrinkage depression were reduced considerably, and in a casting, which requires chill mould the solidifying period was reduced by the addition of the refining agent of the invention without necessitating the chill mould and a large and complicated casting requires only heating at 650 C. for relieving of casting stresses to provide commercial products so that there is advantage that the operating step is shortened and faulty castings can be prevented possibly to attain the reasonable production by the invention.

TEST EXAMPLE 3 Heat Resisting Cast Steels Object: Testing method of the prevention of coliunnar crystals and segregation as well as economy of heat treatment.

Method of testing:

C 0.45 P 0.03 Si 1.80 S 0.025 Mn 0.80 Cr 30.00

The heat resisting cast steel of high chrome content having the above mentioned composition was melted in 2 tons electric furnace (basic) and the refining agent of the invention was added to 1 ton of the molten steel when taken in a ladle at a rate of 0.08% of the agent by weight of the melt and allowing the reaction time of 3 to 5 minutes after the addition, the melt was poured into moulds of the product and test piece as shown in FIGS. 8(a) and (b) and the remaining 1 ton of the melt was poured into the same test pieces under the same condition without adding the refining agent of the invention.

RESULTS Flziidity.The molten steel treated by the invention showed very improved fluidity in case of pouring into the mould as shown in FTGS. 8(a) and (b), while the untreated molten steel showed oxidizing film when the melt was filled in a riser, whilst the treated sample did not show such oxidizing film.

Fracture iest.The riser of the cast product as shown in FIGS. 8(a) and (b) was cut off at a point adjacent to the main body to compare the fractures, the results of which are shown in FIGS. 9(a) and (b). As seen from FIGS. 9(a) and (b), the sample treated by the invention showed uniform fracture and dispersed columnar crystals Whencompared with that of the untreated samples which show the development of large columnar crystals. The high chrome cast steels have unavoidably produced a columnar structure and it was diificult to perfectly dilfuse such structure even by subjecting the casting to repeated heat treatments, whilst if 0.08% of the refining agent of the invention is added into a ladle the columnar structure was absent in the as-cast condition, and moreover, shrinkage depression was reduced and the operation steps were simplified and all kinds of faults could be possibly avoided.

TEST EXAMPLE 4 Stainless Cast Steel Object: Method of testing the prevention of columnar crystal and segregation as well as shortening the heat treatment.

Method of testing:

C 0.05 S 0.01 Si 0.70 N1 8.50 Mn Q. 1.50 Cr 18.80 P 0.02

The stainless cast steel having the above mentioned composition was melted in 1 ton high frequency electric furnace and the refining agent of the invention at a rate of 0.10% by weight was added to about 300 kg. of molten steel and 0.2% was added to another 300 kg. of the melt and the remaining 400 kg. of the melt without treatment were poured into the moulds of each product to compare the eifect on the structure.

The test pieces were poured into metal mould of 40 mm. dia. at room temperature.

Results:

Fluidity.-The samples which were added with 0.10% and 0.20% of the refining agent of the invention respectively showed the reaction phenomena simultaneously to the addition and the fluidity was considerably improved, whilst the sample without treatment showed the poor fluidity compared with the former and the oxidized film appeared.

Casting structure-The test pieces made in metallic moulds of 40 mm. dia. were cut from the central part to compare the fractures, the results of which were as follows:

In test piece added with 0.20% of the refining agent of the invention the columnar crystal has substantially disappeared, while that added with 0.l0% showed the columnar structure intensely.

Fractures due to sand moulds-The molten steel of the above composition was poured into sand moulds and the risers of about 40 mm. dia. products were cut off, the results of which are shown in FIGS. 10(a), (b) and (c).

In the test pieces made of sand moulds the columnar crystal was perfectly dissipated even at 0.10% and in the stainless cast steels it was sufficiently effective at less than 0.10% addition.

Purity.The impurities in the grains were inspected, the results of which showed that in the sample of the invention impurities did not accumulate at the central portion of the casting as in the former cases due to the eifect of preventing the above described oriented crystals and of shortening the solidifying period and by scattering and dispersing the impurities, thereby considerably reducing the losses in the mechanical properties due to the concentration of impurities.

As above described by adding a very small quantity of the refining agent of the invention to the alloy cast steel the fluidity is remarkably improved, thereby eliminating cold shut and rough skin caused by the poor fluidity of such kind of cast steel and absence of columnar crystals in the as-cast structure, thereby providing homogeneous and the sample (K-223) which was not added with the refining agent of the invention was compared with that (Ii-224) added with the refining agent, both samples being moulded in square moulds.

(b) The refining agent of 0.044% was added immediately before the steel was tapped.

(c) Tapping temperature, 1,510 C.

(d) Composition:

CSi Mn Cr NiP S K-223 0. 09 0. 73 1. 22 17. 75 9. 19 0. 034 0. 011 K424 l0 76 1. 18 17. 92 9. 29 O36 012 RESULTS (1) Mechanical test:

(a) Samples.Steel ingots were split along the longitudinal direction into four pieces to prepare the samples and quenched in water at 1,l00 C. (hardness, K-223 was 149, K-224 was 143).

(2) Swaging test:

(a) SampZes,Steel ingots were split longitudinally in to four pieces and test pieces of mm. dia. and mm. dia. were taken from each top and bottom.

. (b) T est dam-The test pieces were heated at 1,000 C. and swaged to the height of 10 mm. (time of swaging, 7 seconds, and number of hammering, 22 repetitions).

(c) Results.ln the sample without addition, there occurred cracks at one place on the outer periphery of the top sample and at four places for the bottom sample.

The samples added with the refining agent no cracks occurred in all of the test pieces.

What I claim is:

A method of improving the quality of irons and steels, which comprises adding a compound consisting of Mg (BO and Mg(BO to a melt of cast irons and steels while in the molten state in a ladle at a rate of 0.001 to 1% by weight of said melt for improving and nodularizing the casting structure.

References Cited in the file'of this patent UNITED STATES PATENTS 2,751,292 Bogart et June 19, 1956 

0.001 TO 1% BY WEIGHT OF SAID MELT FOR IMPROVING AND NODULARIZING THE CASTING STRUCTURE. 